[minima] Re: Q6 7 8 Amp - gain?
- From: <jerry@xxxxxxxxxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 20 Jan 2014 03:39:37 -0700
Farhan, what transistors did you use? I used 3904's.
-------- Original Message --------
Subject: [minima] Re: Q6 7 8 Amp - gain?
From: Ashhar Farhan <farhanbox@xxxxxxxxx>
Date: Mon, January 20, 2014 4:25 am
To: minima@xxxxxxxxxxxxx
.. kidding. but, the long explanation is this:
If you read the kopski/hayward article, there is a table towards the
bottom that gives the substitute resistances for various gains. I chose
to use 1000 ohms for feedback and 10 ohms for degeneration, just because
those are the ones I had in my junk box.
Then, as this was a tx if amp, I figured it could do with higher
standing current in the output transistor. So, I swapped the 470 ohms of
the original design with 220 ohms (doubling the standing current).
It should give 16-20 db of gain. the Q6 has 2V on the base, and 1.4v on
the emitter. As the collector load resistor is 220 ohms as well, there
would be 10.6 V on Q6's collector.
10.6v on Q6's collector (and Q7's base) will make Q7's emitter voltage
10V... and Q8's emitter voltage 9.4V. The standing current in Q8 will be
9.4/220 = 42mA.
Adding the other currents will make this about 50 mA, not too far away
from the 60 mA that you Mark measured.
Mark, just check the DC voltages. Next, check the values of the feedback
resistor between the base and emitter of Q6 and the 10 ohm emitter
degeneration resistor .
For it to clock 60 mA, it could be that Q6's collector is open. This
could happen for various reasons. Check, check!
- f
On Mon, Jan 20, 2014 at 3:42 PM, Ashhar Farhan <farhanbox@xxxxxxxxx>
wrote:
20 db.
On Mon, Jan 20, 2014 at 3:15 PM, Steve VK2SJA <vk2sja@xxxxxxxxxxx>
wrote:
Hi Mark,
I should keep my mouth shut to hide my ignorance as I don't really know
what the gain should be but here are a couple of thoughts. Farhan will
chime in with the correct answer shortly no doubt.
That amp is the suggested improved Bitx bidirectional amp by W7ZOI.
There
is a technical paper discussing it in the references section of
Farhan's
Minima web page right down the bottom. As originally suggested it is
intended to be bidirectional with 50 ohm termination on both ports.
So first suggestion is have as read of that document.
Second is that if it is indeed still setup as a 50ohm in and out then
your
measurements on the 50ohm scope would be better if you ditch the 10x
probe
and just use a small length of 50 coax to the scope.
I suspect that amp should gave gain closer to 15-20dB but don't really
know as Farhan seems to have departed from the listed resistor values
suggested by Wes in the article. Have a read and you'll see what I
mean.
http://www.phonestack.com/farhan/bidiamp.pdf
There. Now I shall press send and see a one-liner from Farhan telling
us
exactly what the gain should be. :-)
Steve
VK2SJA
> HI Gang
>
> I've built half of the bi-directional amplifier to the right of the
> diode mixer in the schematic and am seeing very small gain in Q6 7 8.
> The amp comprising these three is drawing 60mA and mine has been
> constructed using MPSH10s (because I have thousands of them!).
>
> Any thoughts on what we expect from this part of the circuit - in the
> attached yellow is in and blue is out. Gain is about 4.5dB - is this
> right? The scope is 50R terminated on both channels and I am using
X10
> probes.
>
> Mark
> G0MGX
>
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